Methods and apparatus for discriminating polymorphic tachyarrhythmias from monomorphic tachyarrhythmias facilitating detection of fibrillation

ABSTRACT

Systems and methods employing a weighted zero crossing sum metric (WZCS M ) derived from the EGM that improves the specificity of discriminating between a monomorphic tachyarrhythmia and a polymorphic tachyarrhythmia are provided that examine frequency content and baseline information of the EGM as discriminatory signatures are disclosed. In preferred embodiments, high rate polymorphic QRS complexes are discriminated from high rate monomorphic QRS complexes to increase the specificity of detection of polymorphic VT and VF. Zero crossing points (ZCPs) and weighted ZCP slopes of the high pass filtered EGM signal in baseline and sense event windows are identified. The weighted ZCPs of the baseline window are summed to provide a baseline WZCS B , and the weighted ZCPs of the VSENSE event window are summed to provide a VSENSE event WZCS E . A WZCS M  is derived from the VSENSE event WZCS E  and the baseline WZCS B  that is compared to a threshold.

RELATED APPLICATIONS

[0001] This application claims priority to provisional U.S. ApplicationSer. No. 60/430926, filed Dec. 4, 2002.

[0002] This application is related to U.S. patent application Ser. No.(P-10794.00) filed on even date herewith for METHODS AND APPARATUS FORDISCRIMINATING POLYMORPHIC TACHYARRHYTHMIAS FROM MONOMORPHICTACHYARRHYTHMIAS FACILITATING DETECTION OF FIBRILLATION in the names ofMark. L. Brown et al. and to U.S. patent application Ser. No.(P-10794.02) filed on even date herewith for METHODS AND APPARATUS FORDISCRIMINATING POLYMORPHIC TACHYARRHYTHMIAS FROM MONOMORPHICTACHYARRHYTHMIAS FACILITATING DETECTION OF FIBRILLATION in the names ofShantanu Sarkar et al.

FIELD OF THE INVENTION

[0003] This invention relates to implantable medical devices (IMDs), andmore particularly to improved methods and apparatus for discriminatingamong tachyarrhythmias in implantable heart monitors and cardiacstimulators, such as implantable cardioverter/defibrillators (ICDs).

BACKGROUND OF THE INVENTION

[0004] By way of definition, the heart is said to be in normal sinusrhythm (NSR) when the atria and ventricles beat in synchrony at a heartrate lower than a defined tachycardia heart rate that providessufficient cardiac output of oxygenated blood to the body at rest andduring exercise or stress. The term bradycardia refers to an abnormalslow rate of one or more heart chamber that inappropriately providesinsufficient cardiac output at rest or during stress or exercise. Theterm “tachyarrhythmia” refers to any abnormal fast rhythm of one or moreheart chamber that reduces cardiac output and may be amenable ofconversion to NSR by “cardioversion” or “defibrillation” or theapplication of certain anti-tachycardia pacing therapies to the heartchamber as described further herein. Atrial tachyarrhythmias includeatrial tachycardia (AT) and atrial flutter or fibrillation (AF)originating from one or more ectopic sites in the right or left atria.Ventricular tachyarrhythmias include ventricular tachycardia (VT) andventricular flutter or fibrillation (VF) originating from one or moreectopic sites in the ventricles. Supraventricular tachycardia (SVT) canalso result from high rate atrial tachyarrhythmias or junctionaldepolarizations conducted to the ventricles including AV re-entranttachycardia, which usually conducts down the AV node and up through leftpostero-lateral bypass tract is considered an SVT. Individuals whosehearts go into VF or into high rate, polymorphic VT can suffer suddencardiac death (SCD) unless the rhythm terminates either spontaneously ortherapeutically within a very short time after onset of such high rateVT or VF.

[0005] AF and VF are characterized by chaotic electrical activity thatexhibits highly variable depolarization wavefronts that are propagatedin directions that differ from the directions of propagation during NSRand more rhythmic tachycardias. The depolarization waves traversing theatria during AF and the ventricles during VF do not follow normalconduction pathways and can vary in direction from beat to beat. DuringAF and VF episodes (particularly at onset and during the initial phasebefore cardiac activity diminishes), the depolarization waveforms areirregular in amplitude and hence in appearance when viewed on anelectrocardiogram strip or display and are characterized as“polymorphic”. In addition, the atrial or ventricular EGM does notexhibit a characteristic baseline of little electrical activityseparating P-waves or QRS complexes, respectively.

[0006] The QRS complexes of rhythmic atrial and ventricular tachycardiaepisodes typically exhibit a regular or “monomorphic” P-waves or QRSwaveforms that simply become narrower as heart rate increases from NSRand that are separated by a baseline interval. However, the QRScomplexes during certain VT episodes can be polymorphic, particularlyfrom one beat to the next. Such polymorphic VT episodes may be due toreentry conduction through diseased tissue, which results in QRSdepolarization wavefronts that are also typically propagated indirections that differ from those prevalent during NSR or monomorphic VTor SVT episodes.

[0007] In the field of automatic implantable arrhythmia control devices,particularly ICDs (also referred to aspacemaker/cardioverter/defibrillators or PCDs), the terms“cardioversion” and “cardioverter” and “defibrillation” and“defibrillator” generally refer to the processes of and devices fordischarging relatively high energy electrical shocks into or acrosscardiac tissue to arrest a life threatening tachyarrhythmia. Inpractice, the conversion of AT or VT or low rate AF or VF to normalsinus rhythm by a relatively low amplitude cardioversion shock deliveredin timed synchrony with a sensed atrial or ventricular cardiacdepolarization (P-wave or R-wave) is typically referred to as“cardioversion”. The conversion of malignant AF or VF by the same orhigher energy shock delivered without requiring such synchronization istypically referred to as “defibrillation”. Synchronization can beattempted, but therapy is delivered without synchronization ifsynchronization is not possible in a short time. Cardioversion shocksthat may or may not be synchronized with a cardiac depolarization orrhythm and may be applied to arrest a VT with a lower range energy pulseof around 1-15 Joules or VF with a medium to high energy pulse of 7-40Joules, nominally. In the following description and claims, it is to beassumed that the terms cardioversion and defibrillation areinterchangeable, and that use of one term is inclusive of the other,unless specific distinctions are drawn between them in the context ofthe use. For convenience, cardioversion and/or defibrillation shocks orshock therapies are referred to herein as C/D shocks or shock therapies.

[0008] Bradycardia cardiac pacing functions are also currentlyincorporated into ICDs to supplant some or all of an abnormal heart'snatural pacing function by delivering appropriately timed pacing pulsesto cause a chamber or chambers of the heart to contract or “beat”, i.e.,to “capture” the heart. Either single chamber (atrial or ventricular)pacing functions or dual chamber (atrial and ventricular) pacing pulsesare applied to the atria and/or the ventricles in response tobradycardia or dissociation of the atrial and ventricular heart rates ata pacing rate to restore cardiac output that is appropriate to thephysiologic requirements of the patient. Most recently, synchronizedright and left heart pacing, particularly synchronized pacing of theright and left ventricles, has been incorporated into ICDs for heartfailure patients who are also susceptible to tachyarrhythymias.

[0009] In addition, anti-tachycardia pacing capabilities have beenincorporated into ICDs for delivering bursts of pacing pulses or singleoverdrive pacing pulses to the atria and/or the ventricles to counterand convert certain slow AT or VT episodes to normal sinus rates. Thenumber, frequency, pulse amplitude and width of burst pacing pulsetherapies can be programmed by remote programming and telemetryequipment to meet physiologic needs of the particular patient and powerconservation requirements.

[0010] Among the most important functions of such ICDs are to detecttachyarrhythmias, to correctly identify the tachyarrhythmia, to supplyan appropriate C/D shock or burst pacing therapy, and to determinewhether or not the supplied therapy was effective.

[0011] The detection criteria first proposed to trigger automaticdelivery of a C/D shock to the ventricles constituted the presence of ahigh ventricular heart rate and/or lowered or absent blood pressuremeasured in the right ventricular chamber. However, chronic bloodpressure sensors were not long-lived and reliable, and simple heart ratedetection itself proved unreliable, resulting in failure to detect trueVF episodes and false declarations of VF due a variety of causes.

[0012] Consequently, one of the techniques implemented in the first ICDsfor determining when defibrillation or cardioversion shocks should bedelivered employed a probability density function (PDF) fordiscriminating VF from VT as disclosed in U.S. Pat. Nos. 4,184,493 and4,202,340. Briefly, the PDF defines the fraction of time, on theaverage, that a given signal spends between two amplitude limits. It isasserted in the '340 patent that the PDF of an EGM changes markedlybetween VF and NSR. Generally speaking, the PDF VF detection criteriaare satisfied when the time-averaged derivative of the EGM remains offthe base line for extended period of time. Accordingly, it is assertedthat VF could be detected by providing a mechanism for generating a PDF(or a portion thereof), or approximating one or more points on thefunction. The entire PDF need not always be developed; rather, it issometimes sufficient to develop only particular values of the functionat certain sampling points. Various circuits for developing andutilizing an entire PDF curve, or for developing the function, andsampling the same only at select points, or for approximating a PDF at asingle point are set forth in the '493 and '340 patents.

[0013] A VF detector is also disclosed in the '340 patent that sensesthe regularity of the R-R interval. It was observed that R-waves canstill be identified during high rate VT episodes (on the order of 250beats per minute) and that the R-R intervals are stable, whereas R-Rintervals vary during VF episodes. Therefore, incorporation of a phaselock loop circuit to monitor the variability in the R-R interval isproposed in the '340 patent to serve as a second-stage detector alongwith the PDF detector. The phase lock loop locks onto the regular R-Rintervals or non-malignant VT or SVT, but the phase lock loop cannotlock onto the irregular R-to-R intervals characteristic of VF. It isasserted in the '340 patent that by utilizing the PDF detector as afirst detector stage and a phase lock loop detector as a second detectorstage, the absence of a locked stage in the phase lock loop detector,coupled with the condition of the first detector stage having declared aVF, verifies the presence of VF with an exceedingly high degree ofaccuracy.

[0014] However, it was found that these proposed uses of the PDF and thephase lock loop were still inadequate as reported in later filed U.S.Pat. No. 4,475,551. It was found that the PDF detector could be“triggered” not only by actual VF, but also by some forms of high rateVT and even low rate VT exhibiting certain abnormal EGM patterns. Thepossibility of such triggering in the presence of high rate VT isconsidered acceptable because high rate VT that significantly lowerscardiac output can be fatal. However, delivery of a defibrillation orcardioversion shock in the presence of non-life threatening, low rate VTcould itself trigger a malignant VF. A number of additional R-R intervaldiscrimination criteria and/or specific sense electrode configurationsare proposed in the '551 patent to facilitate accurately distinguishingVF and high rate VT from non-malignant low rate VT. It appears however,that the PDF became less and less useful as sensing and ratediscrimination capabilities were enhanced.

[0015] The typical VT and VF detection criteria that have been employedin commercially released ICDs employ rate/interval based timingcriterion and duration or frequency criterion as a basic mechanism fordetecting the presence of and distinguishing between tachyarrhythmias.To this end, the intrinsic heart rate is measured on a beat-to-beatbasis by timing the R-R interval between successive ventricular sense(VSENSE) event signals output by an R-wave sense amplifier. The measuredR-R intervals are compared to a fibrillation detection interval (FDI), afast tachycardia detection interval (FTDI) and a slow tachycardiadetection interval (TDI), and respective VF, fast VT or slow VT countsare accumulated depending on the result of the comparison. One of VF,fast VT or slow VT is declared when a count matches a particular numberof intervals required for detection (referred to herein as “NID”). Eachrate zone may have its own defined NID, for example, “VFNID” forfibrillation detection, “FVTNID” for fast VT detection, and “VTNID” forslow VT detection.

[0016] For example, the measured R-R intervals are compared to the FDIcriterion, and the ventricular sensed event is declared a VF event or anon-VF event depending upon the results of the comparison. VF isprovisionally declared when the count meets (i.e., equals or exceeds)the VFNID frequency criterion. Similarly, the ventricular sensed eventcan be declared a fast VT or a slow VT depending on the results of thecomparison to the FTDI and the TDI.

[0017] Often, SVT episodes causing the ventricles to beat at a rate thatmeets the FDI and can be inappropriately detected as VF episodes. InICDs having dual chamber, atrial and ventricular, sensing capabilities,further strategies have been generally followed to detect and classifyatrial and ventricular tachyarrhythmias. Algorithms have been developedthat identify atrial sensed events from P-waves and/or ventricularsensed events from R-waves and derive atrial and/or ventricular eventintervals and/or rates therefrom. Various detection and classificationsystems have been proposed as set forth in commonly assigned U.S. Pat.Nos. 5,342,402, 5,545,186, 5,782,876, and 5,814,079, that invoke ahierarchy of prioritization rules to make a decision as to when acardioversion/defibrillation therapy is to be delivered or withheld.These rule-based methods and apparatus have been incorporated in dualchamber ICDs to distinguish atrial and ventricular tachyarrhythmiasemploying “PR logic” in dual chamber MEDTRONIC® GEM® DR ICDs.

[0018] Single chamber ICDs for distinguishing VF from VT or SVT andproviding ventricular C/D shock therapies and/or burst pacing therapiesdo not have the capabilities of sensing P-waves to detect atrial sensedevents and analyzing the relationship between atrial sensed events andventricular sensed events based on detected R-waves. Therefore, manyother proposals have been made to examine electrogram (EGM) waveformcharacteristics, particularly unique waveform characteristics of the QRScomplex during NSR, VT, VF and SVT.

[0019] One method of discriminating between VF and NSR EGM waveforms asset forth in commonly assigned U.S. Pat. No. 5,312,441, for example, isbased on measurements and comparisons of the width of the ORS complex toVF width criterion. A normal ORS complex is generally narrower than theabnormal QRS complex during VF, and so QRS width can be employed todistinguish the normal QRS complex from the abnormal QRS complex duringVF. However, there are cases when an abnormal QRS complex during VT willhave a different morphology than the normal QRS complex while remainingnarrow. Conversely, the QRS complex during certain SVT episodes can alsobe wide. In those cases, a more sensitive and selective method is neededto discriminate between different waveforms.

[0020] As noted above, QRS depolarization waves traversing theventricles during VF do not follow normal conduction pathways and canvary in direction from beat to beat, whereas QRS depolarization wavesduring SVT that follow normal conduction pathways or during VT emanatingfrom stable ectopic depolarization sites do not vary significantly indirection. Therefore, various proposals have been made to distinguish VFfrom a stable VT or SVT as a function of the QRS wave propagationdirection on a beat-to-beat basis.

[0021] The VT/VF discriminator disclosed in commonly assigned U.S. Pat.No 5,193,535 employs two sense electrode pairs, e.g., a near field orbipolar electrode pair and a far field or unipolar electrode pair, thatare coupled to detection circuitry for identifying the points in timewhen the sensed electrical signals resulting from the passage of adepolarization wavefront (QRS complex) meet certain predeterminedcriteria, hereafter referred to as the first and second “fiducialpoints”, that may or may not be the same. The cumulative variability ofthe time intervals separating the occurrence of the first and secondfiducial points over a series of R-R intervals that satisfy VF or VTdetection criteria is determined. In general terms, the cumulativevariability of a series of true VF QRS complexes resulting insatisfaction of VF detection criteria is higher than the cumulativevariability of a series of stable VT QRS complexes or SVT QRS complexessatisfying the VF detection criteria. The cumulative variability valueor index is used to distinguish VF from high rate VT to trigger orwithhold delivery of a C/D shock therapy. Similar techniques aredisclosed in U.S. Pat. No. 5,810,739.

[0022] A further approach to the discrimination of normal heart beatsfrom abnormal heart beats employing the morphology of the QRS complex isbased on making a comparison of the waveform of the ORS complex duringtachyarrhythmia with the waveform of a “template” recording of a QRScomplex in NSR and optionally, other template recordings made during VFor VT. An ICD is disclosed in commonly assigned U.S. Pat. No. 5,447,519that discriminates between monomorphic ventricular tachyarrhythmias,particularly VT, from polymorphic ventricular tachyarrhythmias,particularly VF. A fiducial point of each successive QRS complex isdetected (e.g., a VSENSE) prompting the storage of sampled and digitizedwaveform amplitude data within a timing window bridging the point intime of fiducial point detection. Stored sets of such sampled wave shapedata are compared data point to data point resulting in a sampledmorphology index value for each compared set. The magnitude of thesampled morphology index value or a series such index values areanalyzed to determine the presence of a single or a progression ofbeat-to-beat waveform changes indicative of a polymorphic singletransition or progression of QRS complexes from monomorphic topolymorphic waveforms indicative of an arrhythmia that should be treatedwith aggressive C/D shock therapies. The ICD is preferably provided witha closely spaced and widely spaced pairs of electrodes for sensing eachQRS complex as in the above-referenced '535 patent. The closely spacedelectrode pair is coupled to sense detect circuitry for identifying thefiducial point and to counting and comparison circuitry for developingrate and onset data. The widely spaced pair of electrodes is coupled tosense and digitizing circuitry for developing the sampled waveformamplitude data from which the morphology index values are derived.

[0023] The common approach for such morphology analysis is CorrelationWaveform Analysis (CWA) or its less computationally costly counterpart,so-called Area of Difference (AD) analysis. Both require minimization ofa function describing difference between two signals (sum of squareddifferences of wave data points for the case of CWA, and the sum ofabsolute values of the differences for AD analysis). However, suchcomputations, as typically performed, are more computationally costlyand consume more power to carry out than is generally desirable withinICDs.

[0024] Uses of the Haar wavelet transform for performing morphologicanalysis and discrimination of normal and abnormal QRS complexes isdescribed in U.S. Pat. No. 5,439,483 and in commonly assigned U.S. Pat.No. 6,393,316. The '316 patent discloses a method and apparatus forreliable discrimination between ventricular depolarizations resultingfrom normal and abnormal propagation of depolarization wavefrontsthrough the chambers of a patient's heart by means of a Haar wavelettransform-based method of analysis of QRS complexes of the EGM. Severalembodiments are described in the '316 patent that involve thedevelopment of WTC templates of NSR as well as SVT QRS complexes andcomparison of current high rate QRS complexes satisfying VT or VF ratecriteria to the stored WTC templates. Certain features of the waveletmorphology algorithms disclosed in the '316 patent are employed in thesingle chamber MEDTRONIC® Marquis® VR ICDs.

[0025] A method and system are provided for monitoringelectrocardiographic signals and detecting a pathological cardiacarrhythmia is disclosed in U.S. Pat. No. 5,000,189, wherein the zerocrossings of the first derivative of a reference template (i.e.reference waveform) are utilized to separate or partition both thetemplate and each subsequent electrocardiographic signal being monitoredinto first and second sets of identifiable partitions. Each zerocrossing is a boundary between adjacent partitions. Initially, thereference template is generated by acquiring a first set of waveformdata representing a known good electrocardiographic signal. Identifiablepartitions of the first set of data are then matched with correspondingidentifiable partitions of the second set of data to obtain aperformance measure signal. In one embodiment, the area beneath thederivative in each partition of the analyzed waveform is computed andcompared (i.e. matched) to the corresponding area of the template.Preferably, a plurality of electrocardiographic signals is analyzed byutilizing the template and a plurality of performance measure signalsare obtained. Finally, a therapy signal is provided as a function of theplurality of performance measure signals in the event of a pathologiccardiac arrhythmia.

[0026] Both the complexity and the indications for implantation of theabove-described ICDs have increased remarkably over the years. Patientswho receive such ICDs are typically identified as survivors of SCDsecondary to VF that may originate as VT. In such cases, the cost andcomplexity of such ICDs is deemed warranted. However, many patientslikely to suffer SCD are presently un-diagnosed and do not survive theirfirst VF episode. It is believed that certain patient populations existthat could be identified from other indicia and could benefit from a“prophylactic”, low cost, limited function, ICD that simply providesprotection against SCD due to VF. To minimize cost of the ICD and theimplantation procedure, such a prophylactic ICD would necessarily havelimited functions and the capability of delivering high-energy C/Dshocks in response to a detected VF episode.

[0027] In a prophylactic ICD application, there is concern that theselected patients will exhibit nearly the same frequency of SVT episodesbut far fewer polymorphic VT or VF episodes than is exhibited by theconventional ICD patient population. Therefore, it is feared that theuse of the current VF detection algorithms will result in a higherpercentage of inappropriate VF shock therapies than in the conventionalICD population. This is expected because Bayes' theorem teaches thatdetection performance depends not only on the detection algorithm'sintrinsic performance, but also depends on the population oftachyarrhythmias that the algorithm processes.

[0028] In the prophylactic ICD application, AF episodes that conductrapidly to the ventricle (rapidly conducted AF) are of particularconcern. The ventricular rate of such AF events is often similar to thatof VF and very hard to discriminate from simultaneous AF and VTNF on thebasis of intervals alone. Wilkoff et al. identified rapidly conducted AFas one of the primary algorithmic causes for inappropriate VTNFdetection in dual-chamber ICDs. In a single chamber detection scenariofor a wider population, as in the case of prophylactic ICDs, it isexpected that rapidly conducted AF that conducts at ventricular ratesthat overlap with the VF zone will also be a primary algorithmic causefor inappropriate detection. See, Wilkoff B. L. et. al., “CriticalAnalysis of Dual-Chamber Implantable Cardioverter-DefibrillatorArrhythmia Detection: Results and Technical Consideration”, Circulation,2001;103:381-386.

[0029] The ORS morphology during rapidly conducted AF often differs fromthe ORS morphology during NSR rendering algorithms that rely on afinding of similarity between the current QRS complex morphology and anNSR QRS complex morphology to distinguish SVT from VF much lesseffective. Although the QRS morphology during AF episodes differs fromNSR QRS morphology, there is often a characteristic QRS complexmorphology during AF that is relatively stable over short periods oftime.

[0030] Therefore, a need remains for a robust and computationallyefficient VF detection capability of discriminating a true VF episodefrom high rate VT or SVT that is not life threatening particularly foruse in prophylactic ICDs to avoid the unnecessary delivery of a C/Dshock therapy. Such a VF detection capability would, of course, still bebeneficial in more complex single chamber, dual chamber andmulti-chamber ICDs. Such a robust VF detection capability may also findutility in an implantable heart monitors (IHM) having a sense electrodearray (SEA) implanted subcutaneously for monitoring, processing, andstoring data from the EGM sensed across one or more selected far fieldsense vector as described in commonly assigned U.S. Pat. Nos. 5,331,966,for example.

[0031] Moreover, a need remains for a robust and computationallyefficient AF detection capability of discriminating a true AF episodefrom high rate AT.

SUMMARY OF THE INVENTION

[0032] In an implantable medical device that provisionally detects apolymorphic tachyarrhythmia of the heart of a patient as a function ofmeasured time intervals between sensed events in a cardiac signal,systems for and methods of improving the specificity of discriminatingbetween a monomorphic tachyarrhythmia and a polymorphic tachyarrhythmiaare provided that examine frequency content and baseline information ofthe EGM as discriminatory signatures. The present invention ispreferably employed in a prophylactic single chamber ICD or in a morecomplex single chamber, dual chamber or multi-chamber ICD or in cardiacmonitors.

[0033] In accordance with one embodiment of the present invention,methods and apparatus are provided for discriminating high ratepolymorphic QRS complexes from high rate monomorphic QRS complexes toincrease the specificity of detection of polymorphic VT and VF fromother rapid ventricular contractions that can result from AF that israpidly conducted to the ventricles resulting in R-R intervals thatsatisfy VT/VF rate detection criterion. In particular, methods andapparatus are provided that robustly distinguish VF or polymorphic VTfrom monomorphic VT and rapidly conducted AF. The invention may also beapplied to discriminate atrial tachyarrhythmias, particularly AF fromAT, through examination of frequency content and baseline information ofthe atrial EGM.

[0034] In an exemplary ICD embodiment, the methods and apparatus of thepresent invention augment VF detection criteria by determining if apredetermined number of high rate QRS complexes resulting in detectedventricular sense events and satisfying the VF detection criteria aremonomorphic or polymorphic. The delivery of the C/D shock that would bedelivered based on satisfaction of the VF detection criteria iswithheld, and an anti-tachycardia therapy may be delivered if thepredetermined number of the ORS complexes have high frequency content(rapidly varying signals) during the QRS complexes as compared to thebaseline region between the present QRS complex and the previous one. Inother words, a sequence of high rate QRS complexes that satisfied the VFdetection criteria are deemed to be more likely due to VT or SVT thandue to VF if at least a number of the QRS complexes are determined tohave high frequency content and baseline segments with low frequencycontent between them. In an embodiment that further increasesspecificity, delivery of the C/D shock is postponed by a withhold delaynumber (e.g., z) of subsequent ventricular sensed events each time thata QRS complex is examined and the predetermined number of ORS complexesare found to satisfy the frequency content criteria.

[0035] Preferred embodiments of the present invention use a measure ofthe frequency content of the ORS complexes with respect to baseline as asupplementary VF discriminatory criterion to aid in discrimination of VFand polymorphic VT from monomorphic VT and SVT. A particular algorithmof the present invention that determines frequency content of the QRScomplexes with respect to a baseline segment is titled the Weighted ZeroCrossing Sum (WZCS) algorithm. A WZCS metric is derived from the currentQRS complex that reflects frequency content information and baselineinformation. A match count x of a running series y of QRS complexes canbe incremented or decremented when the WZCS metric meets or does notmeet, respectively, the WZCS threshold, depending on whether the ythWZCS metric met or did not meet, respectively, the WZCS threshold. TheWZCS count x of y is compared to a count threshold, and the QRScomplexes are deemed to be more or less likely to signify VF andpolymorphic VT depending upon the results of the comparison. Thus, the“morphologic stability” is a function of the stability of the frequencycontent and baseline information of the series of QRS complexes. In thiscase, the frequency content difference between QRS complexes andbaseline segments is used as a morphological marker.

[0036] In general, the WZCS metric (WZCS_(M) ) is derived from thesampled, digitized and high pass filtered EGM. Baseline windows betweenVSENSE events and VSENSE event windows embracing the QRS complex aredefined, and zero crossing points of the high pass filtered EGM signalare identified in each window. The absolute value of the slope of thehigh pass filtered EGM signal is determined at each zero crossing point(ZCP), and each zero crossing point is weighted by the correspondingdetermined slope. The weighted zero crossing points of the baselinewindow are summed to provide a baseline WZCS_(B), and the weighted zerocrossing points of the VSENSE event window are summed to provide aVSENSE event WZCS_(E). The WZCS_(M) is derived from the VSENSE eventWZCS_(E) and the baseline WZCS_(B)

[0037] The WZCS_(M) is simple to compute upon each VSENSE event and tocompare to a WZCS threshold based on the WZCS_(M) derived during NSR.The WZCS_(M) is high during high rate monomorphic VT or SVT because theWZCS_(E) substantially exceeds the WZCS_(B) even when the VSENSE eventwindows and baseline windows are closely spaced. Conversely, theWZCS_(E) does not substantially exceed the WZCS_(B) during VF orpolymorphic VT.

[0038] Advantageously, the invention may also be applied to discriminateatrial tachyarrhythmias, particularly AF from AT, through examination ofthe atrial EGM employing the WZCS metric.

[0039] This summary of the invention has been presented here simply topoint out some of the ways that the invention overcomes difficultiespresented in the prior art and to distinguish the invention from theprior art and is not intended to operate in any manner as a limitationon the interpretation of claims that are presented initially in thepatent application and that are ultimately granted.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] These and other advantages and features of the present inventionwill be more readily understood from the following detailed descriptionof the preferred embodiments thereof, when considered in conjunctionwith the drawings, in which like reference numerals indicate identicalstructures throughout the several views, and wherein:

[0041]FIG. 1 is a schematic illustration of an ICD IPG and associatedICD leads extending from the ICD IPG to C/D and pace/sense electrodeslocated in operative relation to the ventricles of a heart;

[0042]FIG. 2 is a schematic block diagram of the circuitry of the ICDIPG of FIG. 1 in which the present invention may advantageously bepracticed;

[0043]FIG. 3 is a flow chart illustrating a system and method ofdetecting and declaring a VF episode and providing a C/D shock therapyor monomorphic fast VT and providing an appropriate therapy inaccordance with one embodiment of the present invention;

[0044]FIG. 4 is a flow chart illustrating a system and method ofdetecting and declaring a VF episode and providing a C/D shock therapyin accordance with a further embodiment of the present invention;

[0045]FIG. 5 is a graphical illustration of WZCS signal processing ofQRS complexes in the EGM; and

[0046]FIG. 6 is a flow chart of the steps of the WZCS algorithm that canbe practiced in the flow chart of FIGS. 3 and 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

[0047] In the following detailed description, references are made toillustrative embodiments of methods and apparatus for carrying out theinvention. It is understood that other embodiments can be utilizedwithout departing from the scope of the invention. In particular, thepresent invention is described in the context of a simple single chamberICD for providing the functions of monitoring the ventricular EGM,detecting VF, ST, and SVT, discriminating VF from VT and SVT, andproviding a C/D shock in response to a detected VF episode, storing datarelated to detected VF, VT and SVT episodes for uplink telemetrytransmission to external medical devices, and optionally providing VVIpacing for bradycardia. The preferred embodiment can advantageously besimplified to function as a prophylactic ICD without pacing capabilitiesthat provides unsynchronized, high energy, C/D shocks upon detection ofVF episodes in anticipation that the patient will thereby survive suchVF episodes and be a candidate for implantation of a more complex ICD.However, it will be appreciated from the following that the variousembodiments and principles of the present invention may be employed andpracticed in an IHM for simply monitoring the ventricular EGM, detectingVF, VT, and SVT, discriminating VF from VT and SVT, and storing datarelated to detected VF episodes for uplink telemetry transmission toexternal medical devices or practiced in more complex tiered therapydelivery, dual chamber or multi-chamber ICDs.

[0048] Although the preferred embodiments described above relate to thediscrimination of ventricular tachyarrhythmias, it will be understoodthat the principles of the present invention may be applied todiscrimination of atrial tachyarrhythmias.

[0049]FIG. 1 illustrates one embodiment of an ICD comprising an ICDimplantable pulse generator (IPG) 10 in which the discriminationalgorithms of the present invention can be advantageously incorporatedand the associated ICD medical electrical leads 12 and 14 extending to ahuman heart 30. The ICD of FIG. 1 is also shown in relation to anexternal programmer 40 and external programmer telemetry antenna 42providing uplink telemetry (UT) and downlink telemetry (DT)transmissions with an IPG antenna.

[0050] The ICD IPG 10 is formed of a hermetically sealed enclosure 16containing the electronic circuitry and components, including a battery,depicted in FIG. 2 and a connector block 18. The proximal ends of theillustrated ICD leads 12 and 14 are inserted into two connector ports ofthe connector block 18 to make electrical connections between leadconductors of the ICD leads 12 and 14 and the circuitry within thehermetically sealed enclosure 16 via feedthroughs extending through theenclosure wall in a manner well known in the art.

[0051] The ICD IPG 10 is intended to be implanted subcutaneously remotefrom the heart, and at least an uninsulated portion of the hermeticallysealed enclosure 16 may be employed as an indifferent pace/sense and/orC/D electrode 20. The ICD lead 14 and the ICD lead 12 are a coronarysinus (CS) lead and a right ventricular (RV) lead, respectively that areextended transvenously from the ICD IPG 10 into the heart chambers usingconventional implantation techniques.

[0052] The CS lead 14 supports an elongated wire coil, C/D electrode 32that is located in the coronary sinus and great vein region of the heart30. The C/D electrode 32 is advanced through the coronary sinus ostiumin the right atrium and around the heart, and is disposed in proximitywith the left ventricular wall either in the great vein or in thecoronary sinus.

[0053] The RV lead 12 supports proximal and distal, elongated wire coil,C/D electrodes 22 and 28, a ring-shaped pace/sense electrode 24, and ahelical pace/sense electrode 26 comprising an active fixation helix. Thehelical pace/sense electrode 26 is screwed into the tissue of the rightventricle at the right ventricular apex to fix the pace/sense electrodes24 and 26 in the right ventricle. Other RV fixation mechanisms wellknown in the art, e.g., soft, pliant tines, may be substituted for theactive fixation helix.

[0054] The C/D electrodes 22 and 28 are disposed in the RV and superiorvena cava (SVC) respectively to define one C/D vector between the baseand apex of the heart 30. An RV-LV C/D vector is defined between the C/Delectrodes 22 and 32. Other C/D vectors can be defined between thesubcutaneous housing electrode 20 and any of the C/D electrodes 22, 28and 32. Pairs of the C/D electrodes 22, 28 and 32 can be selectivelycoupled together to define further C/D vectors in a manner known in theart.

[0055] In conjunction with the present invention, the illustrated ICDleads and described electrodes are merely exemplary of possible leadsystems and electrodes that can be paired together to detect R-waves, toprocess the EGM, to deliver C/D shocks in response to a confirmed VFdetection, and to provide pacing, particularly to the RV. Theillustrated ICD leads and electrodes provide a variety of senseelectrodes that can be paired and coupled to a ventricular senseamplifier to detect R-waves, an EGM amplifier to sense the EGM, and to aC/D shock generator to deliver monophasic or biphasic C/D shocks to theheart to counter VF. It will be understood that other ICD leads andpace/sense and C/D electrodes can be employed in the practice of theinvention as long as the electrodes provide sense electrode pairs fordetection of R-waves, for sensing the EGM, and for delivering themonophasic or biphasic C/D shocks to the heart to counter VF.

[0056] For example, in the simplest case of a low cost, limitedfunction, prophylactic ICD, the ICD leads may comprise a simpler RV leadsupporting only the C/D electrode 22 and a single distal pace/senseelectrode or a bipolar pair of distal pace/sense electrodes. A highenergy C/D shock can be delivered between the C/D electrode 22 and thehousing C/D electrode 20. The R-waves and the EGM can be sensed betweenthe selected pace/sense electrode pairs. RV pacing during bradycardiamay or may not be provided between a selected pace/sense electrode pair.

[0057] Returning to FIG. 1, ring electrode 24 and tip electrode 26 maybe paired together and coupled to an R-wave sense amplifier to detectthe occurrence of an R-wave, and ring electrode 24 and subcutaneoushousing electrode 20 or one of the C/D electrodes 22, 28 and 32 may bepaired together for sensing the EGM signal. Alternatively, pace/senseelectrodes 24 and 26 may be used for both R-wave detection and EGMsensing. Moreover, two of the C/D electrodes 32, 22 and 28 may be pairedtogether for sensing the EGM signal.

[0058] The ICD IPG 10 preferably comprises an ICD operating system asdepicted in FIG. 2 that provides the operating modes and functions ofthe MEDTRONIC® GEM 7227 single chamber ICD IPG, for example, that isprogrammable in operating mode and parameter values and isinterrogatable employing the MEDTRONIC® Model 9790C external programmer40, for example. FIG. 2 is a functional block diagram illustrating sucha single chamber ICD operating system 100 that is merely exemplary of avariety of single chamber and dual chamber ICD systems having all orsome of the capabilities described above in which the VTNFdiscrimination functions of the present invention can be advantageouslyimplemented. Moreover, the present invention can be incorporated in animplantable monitor having selected components of the operating systemof FIG. 2.

[0059] The programming of ICD operating modes and parameters or theinterrogation of data stored in the ICD IPG 10 or the initiation of UTtransmission of the real time cardiac EGM is accomplished or initiatedvia programming or interrogation commands transmitted in a DTtransmission by programmer 40 from the external telemetry antenna 42 toan ICD telemetry antenna 36 shown in FIG. 2. In the context of thepresent invention, the ICD operating system stores VTNF detectionepisode data and VF delivery data that can be UT transmitted to theexternal programmer 40 for review by a physician. The ICD IPG telemetrysystem decodes the commands in the DT transmission, retrieves andformats the responsive data or cardiac EGM and conveys it to theexternal programmer 40 as an UT transmission in any of the manners knownin the art.

[0060] The ICD system 100 includes one of more ICs typically mounted onone or more hybrid circuit, a PC board mounting a number of discretecomponents, and further large scale, discrete components. The heart ofthe ICD operating system is in hardware and software in themicrocomputer based timing and control system IC 102 that is coupledwith the other system blocks. The system IC 102 comprises the typicalcomponents of a microcomputer with operating algorithms maintained inmemory or embedded in firmware and further operating system controlcircuitry that is conveniently located with it. Various depicted signaland control lines interconnecting these blocks, but not all are shownfor simplicity of illustration and because they play no material role inthe practice of the present invention.

[0061] The large scale, discrete, off-board, components illustrated inFIG. 2 include one or more batteries 136, HV output capacitors 138, 140,and (optionally) housing mounted, patient alert sound transducers 129and/or activity sensors 134. The discrete components mounted to the PCboard include telemetry antenna 36, reed switch 130, crystal 132, a setof HV discrete components of the HV C/D output circuitry 108, andswitching and protection circuit components of isolation, protection andelectrode selection circuitry 114. These discrete components are coupledto system IC 102 through other ICs and hybrid circuits incorporating thefunctional blocks 104-128 and 176 described further below. A similar ICDoperating system to that depicted in FIG. 2 in which the presentinvention can be implemented is disclosed, for example, in theabove-referenced '316 and '535 patents. The depicted functional blocksand discrete components of FIG. 2 can be arranged as part of one or twoLV hybrid circuits, a HV hybrid circuit and a discrete component PCboard. However, it will be understood that a single hybrid circuit couldbe employed that incorporates and supports all of the system ICs.

[0062] The exemplary ICD operating system 100 of FIG. 2 is powered bythe battery 136 coupled to power supplies in power source block 106 fordeveloping regulated high and low voltage power supplies Vhi and Vlothat are supplied to selected ones of the other functional blocks.Preferably, battery 136 is a lithium silver vanadium battery that can beemployed to provide HV capacitor charging current and that produces avoltage from about 3.2 volts when fresh to about 2.5 volts at specifiedend of service for a single chamber ICD and twice these values for adual chamber ICD. Power supply 106 also includes a power-on-reset (POR)circuit that generates a POR signal initially when the battery 136 isconnected with power supply 106 and any time that the voltage of battery136 falls below a threshold voltage.

[0063] The crystal oscillator circuit 120 is coupled to clock crystal132 and provides one or more system XTAL clock that is applied to themicrocomputer-based control and timing system IC and distributed toother blocks of FIG. 2 as appropriate.

[0064] The telemetry I/O circuit 124 coupled with the IPG telemetryantenna 36 includes a UT transmitter that receives formatted UPLINKsignals for uplink transmission and a DT receiver that receives andforwards DOWNLINK signals to telemetry I/O registers and controlcircuitry in system IC 102. In one telemetry scheme known in the art,the telemetry I/O circuit 124 is enabled to receive and decode DTinterrogation and programming commands when the reed switch circuitprovides the RS signal upon closure of reed switch 130 by an externalprogramming head magnetic field. Downlink telemetry RF signals ring anL-C tank circuit including the IPG telemetry antenna 36. Other pacingfunctions are also affected when a magnetic field closes the reed switch130 and the RS signal is generated in a manner well known in the art. Inmore recent telemetry schemes, the reed switch is not employed toreceive DT transmissions and the telemetry antenna can be physicallylocated outside the hermetically sealed enclosure. The components,operating modes and type of telemetry scheme employed in FIGS. 1 and 2are not material to the present invention.

[0065] Optionally, a rate response circuit 122 is coupled to aphysiologic activity sensor 134, which is preferably a transducer oraccelerometer mounted to the IPG housing inner surface and providesactivity correlated output signals to the rate response circuit 122 in amanner well known in the art. The rate response circuit 122 develops arate control parameter (RCP) that is used to vary a pacing escapeinterval to pace the heart at a rate that provides adequate cardiacoutput. The signal processing of the transducer output signal by therate response circuit 122 can be programmed through rate responseparameter commands to develop the RCP in a number of ways known in theart. The RCP associated with a detected VTNF episode can also be storedin memory in the system IC 102 for UT transmission of the episode datato the external programmer 40 for analysis by the patient's attendingphysician.

[0066] Optionally, a patient alert driver circuit 166 is coupled to asound emitting transducer 129, which is mounted adjacent to the interiorsurface of the IPG housing and is powered to emit audible warningsignals in high urgency and low urgency tones to alert the patient of VFdetection and imminent delivery of a C/D shock or of events orconditions of concern warranting physician intervention. The warningsthat can be programmed into operation or programmed “off” includepace/sense and CV/DEFIB lead impedance out of range (too high or toolow), low battery voltage, excessive charge time for charging the HVcapacitors, all therapies in a programmed group of therapies exhaustedfor a given episode, and an indication of the number of shocks deliveredin an episode”.

[0067] The block diagram of FIG. 2 depicts six input/output terminalslabeled V+, V−, I, HVA, HVB, and COMMC that represent the connectorterminals within the IPG connector block 104 that can be coupled to leadconnector elements and lead conductors extending to the respectiveelectrodes 24, 26, 30, 22, 32, and 28. As noted above, the number ofinput/output terminals and associated electrodes can be reduced to theminimal number necessary to practice the present invention.

[0068] Electrode selection switches in the isolation, protection andelectrode selection circuitry 114 selectively couple pairs of the sixinput/output terminals labeled V+, V−, I, HVA, HVB, and COMMC to theR-wave sense amplifier 126, the ventricular EGM amplifier 128 and theV-PACE pulse generator 112 in response to a corresponding sense/paceelectrode selection command from the microcomputer-based control andtiming system IC 102. The sense/pace electrode selection command isprogrammable by the patient's attending physician through use of theexternal programmer 40 as described above.

[0069] A ventricular pacing function operating in any of the ways thatare well known in the art may or may not be included in a low cost,limited function prophylactic ICD as described above. When the V-PACEgenerator 112 is included as depicted in FIG. 2, it provides V-PACEpulses through the selected pace/sense electrode pair having a pulsewidth and pulse amplitude set by the programmed PPW/PPA commands in aVVI of VVIR pacing mode. A timer in the microcomputer-based control andtiming system 102 times out a programmed VVI pacing escape interval or aVVIR pacing escape interval that varies as a function of the RCP outputby the rate response circuit 122. A V-TRIG signal is generated bymicrocomputer-based control and timing system 102 when the VVI or VVIRescape interval times out and applied to the analog rate limit circuit110, which inhibits erroneous triggering of pacing at an unacceptablyhigh rate in a manner well known in the art. The acceptable V-TRIGsignals are passed through the analog rate limit 110 and trigger thedelivery of the V-Pace pulse by the V-PACE pulse generator 112. The VVIor VVIR escape interval is restarted by a VSENSE generated by theventricular sense amplifier 126 in response to an R-wave.

[0070] ln response to a programming command, the V-PACE pulse generator112 can be coupled through the isolation, protection and electrodeselection circuitry 114 to the V+, V− input/output terminals to bethereby coupled with the pace/sense electrodes 24 and 26 to providebipolar RV pacing. Or, the V-PACE pulse generator 112 can be coupledthrough the isolation, protection and electrode selection circuitry 114to the V− terminal to be thereby coupled with the pace/sense electrode26 and any of the 1, HVA, HVB, and COMMC input/output terminals to bethereby coupled with the respective electrodes 20, 22, 32, and 28 toprovide unipolar RV pacing.

[0071] ln one preferred example, the ventricular sense amplifier 126 iscoupled through the isolation, protection and electrode selectioncircuitry 114 to the V+, V− terminals to be thereby coupled with thepace/sense electrodes 24 and 26 to provide bipolar RV sensing ofR-waves. The ventricular sense amplifier 126 comprises a programmablegain, bandpass amplifier, a threshold setting circuit, and a comparatorfor comparing the bandpass filtered ventricular cardiac signal amplitudeto the threshold. The sensitivity/threshold of the ventricular senseamplifier 126 stored in sensitivity register 176 is programmable by thepatient's attending physician through use of the external programmer 40as described above. The ventricular sense amplifier 126 generates theVSENSE signal when it is not blanked and the amplitude of QRS complexexceeds the ventricular sense threshold, which is typically during therise of the R-wave. The inputs to the ventricular sense amplifier 126are disconnected from the V+, V− terminals by opening blanking switchesin isolation, protection and electrode selection circuitry 114 inresponse to and for the duration of a VBLANK signal generated by aventricular blanking circuit in microcomputer-based control and timingsystem IC 102 upon delivery of a V-PACE pulse or a C/D shock.

[0072] Similarly, the ventricular EGM (VEGM) amplifier 128 is coupledthrough electrode selection switch circuits in isolation, protection andelectrode selection circuitry 114 to a pair of the input/outputterminals selected from input/output terminals V+, V−, I, HVA, HVB, andCOMMC in response to a programmable VEGM vector electrode selectioncommand. The VEGM amplifier 128 filters and amplifies the cardiacsignals and provides the VEGM signals to ADC/MUX 104. In the ADC/MUX104, the VEGM is continually sampled at a sampling frequency of 256 Hz,and the sampled analog signal values are digitized and provided as VEGMDATA to RAM memory registers or buffers in system IC 102 for temporarystorage on a FIFO basis. The temporarily stored VEGM DATA are shiftedinto memory registers within system IC 102 when a tachyarrhythmiaepisode at least partially satisfying the VF detection criteria occursas described further herein.

[0073] Such VEGM DATA can be stored for retrieval in an UT transmissionin memory registers to provide programmable length VEGM strips precedingand following the detection of the arrhythmia and encompassing anydelivery of a VF shock. Due to memory limitations, the stored VEGM DATAmay be discarded and replaced each time a VTNF episode is detected.However, historic episode logs can be compiled and incremented in RAM insystem IC 102 that provide the date, time, type of episode, cyclelength, duration, and identify the last stored EGM DATA.

[0074] The depicted HV C/D output circuit 108 is of the type describedin the above-incorporated '316 and '535 patents comprising a DC-DCconverter and a HV output or discharge circuit for discharging thecharge on the HV output capacitor bank 138 and 140 through selected onesof the C/D electrodes 22, 28, 32 and 20 of FIG. 1. The DC-DC convertercomprises a HV charging circuit, a discrete HV step-up transformer, andthe HV output capacitor bank 138 and 140 coupled to the secondarytransformer coils. The charge on the HV output capacitor bank 138 and140 is selectively discharged through combinations of the leads coupledwith the C/D electrodes 26, 30 and 32 of FIG. 1 via HV switches in theisolation, protection and electrode selection circuitry 114. In aprophylactic ICD of the type described above, the depicted HV C/D outputcircuit 108 develops a high energy, monophasic or biphasic, C/D shockthat is delivered through a selected pair of the C/D electrodes 26, 30and 32 of FIG. 1 via the HV switches in the isolation, protection andelectrode selection circuitry 114.

[0075] The microprocessor within the microcomputer-based control andtiming system 102 operates as an interrupt driven device, under controlof software stored in ROM associated with microprocessor and responds tointerrupts including the VSENSE output of the R-wave sense amplifier 126and the time-out of the VVI or VVIR escape interval. Any necessarymathematical calculations to be performed by the microprocessor and anyupdating of the values or intervals controlled by pacer timing/controlcircuitry within the microcomputer-based control and timing system 102take place following such interrupts. These calculations include thosedescribed in more detail below associated with the VF discriminationmethods of the present invention.

[0076] As described above and in the above-referenced '316 patent, thetypical VT and VF detection criteria that have been employed incommercially released ICDs of the type illustrated in FIGS. 1 and 2employ a rate/interval based timing criterion and an NID frequencycriterion as tachyarrhythmia detection criteria for detecting thepresence of and distinguishing between ventricular tachyarrhythmias. Tothis end, the intrinsic ventricular heart rate is measured on abeat-to-beat basis by timing the R-R interval between successive VSENSEsignals output by the R-wave sense amplifier 126. The R-R interval iscompared to the interval ranges or thresholds established, typically byprogramming, for each of VF, fast VT, and slow VT.

[0077] The VF counter, fast VT counter, and slow VT counter functionlike FIFO shift registers having Y stages each set to “1” or “0” thatcan be implemented in hardware, firmware or software. Each time that acurrent R-R interval is shorter than an interval threshold, a “1”, forexample, is advanced into the first stage of the register, the contentsof each stage is advanced to the next stage, and the “1” or “0” in theYth stage is discarded. Similarly, each time that a current R-R intervalis longer than an interval threshold, a “0”, for example, is advancedinto the first stage of the register, the contents of each stage isadvanced to the next stage, and the “1” or “0” in the Yth stage isdiscarded. Thus, the count X of the corresponding VF counter, fast VTcounter, or slow VT counter is “incremented” if a “1” is advanced intothe initial stage of the register and a “0” is discarded from the Ythstage and “decremented” if a “0” is advanced into the initial stage ofthe register and a “1” is discarded from the Yth stage. The count Xremains the same if the same bit value “1” or “0” is advanced into theinitial stage of the register and is discarded from the Yth stage.

[0078] For example, the R-R interval is simultaneously compared to aprogrammed fibrillation detection interval (FDI), a programmed fasttachycardia interval (FTDI), and a programmed slow tachycardia detectioninterval (TDI). The FDI count X_(VF) is incremented if the R-R intervalis shorter than the FDI and a “0” is discarded from the Yth stage orremains the same if a “0” is discarded from the Yth stage. Similarly, aslow VT count X_(VT) is incremented or remains the same in response toan R-R interval shorter than TDI but longer then the FTDI or the FDI,and a fast VT count X_(FVT) is incremented or remains the same inresponse to an R-R interval longer than FDI but shorter than the FTDI.

[0079] The counts X_(VF), X_(FVT), and X_(VT) that accumulate in therespective VF counter, fast VT counter, and slow VT counter may be usedto signal detection of an associated tachyarrhythmia (VF, fast VT, orslow VT) when the count X_(VF), X_(FVT), or X_(VT) reaches apredetermined value referred to herein as the “number of intervalsrequired for detection” (NID). Each rate zone may have its own definedNID, for example “VFNID” for fibrillation detection, “FVTNID” for fastVT detection and “VTNID” for slow VT detection. Thus, VF is declaredwhen X_(VF)=VFNID, fast VT is declared when X_(FVT)=FVTNID, and slow VTis declared when X_(VT)=VTNID.

[0080] The present invention is directed to increasing the specificityof detection of true VF episodes in instances when the VF detectioncriteria may be mistakenly met by fast VT or SVT particularly due torapidly conducted AF or AFL. The present invention can be practiced inthe context of the exemplary ventricular ICD embodiment of FIGS. 1 and 2when conventional VF detection criteria are met or about to be met and aC/D is to be delivered to the RV to convert the apparent VF to NSR. Itwill be appreciated that the particular details of implementation of theVF detection criteria are not of primary importance. Moreover, it willbe appreciated that the above-described fast VT and slow VT detectioncriteria can be eliminated or altered in the implementation of a simpleprophylactic ICD intended to simply deliver a C/D shock therapy upondetection of a true VF episode.

[0081] In accordance with this preferred embodiment of the presentinvention, the VF detection criteria are augmented when the VF detectioncriteria are met (X_(VF)=VFNID) or, preferably, in the process of beingmet (0<X_(VF)<VFNID) by examining the morphology of a running series ofthe y most recent QRS complexes employing WZCS comparisons to determinepresence of high frequency content in x out of y in a series of QRScomplexes with respect to baseline segments between them. For example,the examination of the frequency content employing the WZCS algorithm ispreferably commenced when X_(VF) is less than (VFNID−y) to derive aWZCS_(M) value. The WZCS_(M) value is compared to a WZCS threshold, anda value of “1” or “0” is shifted into a y-stage WZCS-CNT register when aWZCS_(M) value exceeds or falls below, respectively, the matchthreshold. The count x is the number of “1”s in the y stage WZCS-CNTregister.

[0082] If the WZCS-CNT x equals or exceeds a WZCS count threshold forhigh frequency content evidence, then it is assumed that at least x ofthe last y ORS complexes exhibit high frequency content with respect topreceding baseline segments, suggesting that the most recent QRScomplexes likely are due to monomorphic fast VT or SVT, and finaldeclaration of VF and delivery of a C/D shock are prevented. VF isfinally declared and delivery of a C/D shock is allowed only if the VFdetection criteria are met (X_(VF)=VFNID) and if the WZCS-CNT x does notmeet the WZCS count threshold.

[0083] The above-described method can be employed in an ICD capable ofproviding a C/D shock therapy to counter VF and other appropriatetherapies to counter VT. FIG. 3 depicts the steps of declaring that theventricular tachyarrhythmia satisfying the conventional VF detectioncriteria is either a VF episode (or a polymorphic VT episode) or amonomorphic VT (or SVT) and delivering the programmed therapy. In thisillustrated method of FIG. 3, an evidence counter is set to a number zof QRS complexes when the WZCS-CNT x meets the WZCS count threshold(indicating that x of the y QRS complexes exhibit high frequency contentwith respect to preceding baseline segments). The evidence counter isdecremented from z each time that the WZCS-CNT x does not meet the WZCScount threshold. In this embodiment, if the VF detection criteria aremet (X_(VF)=VFNID), but the evidence count exceeds zero (indicating thatx of the y QRS complexes exhibit high frequency content), then finaldeclaration of VF is not made. Instead, VT is declared, and anappropriate VT therapy is delivered.

[0084] The method of FIG. 3 is preferably modified in the mannerdepicted in FIG. 4 to more stringently increase the specificity that aventricular tachyarrhythmia is a true VF when the ICD is only capable ofdelivering a C/D shock therapy to counter VF. In this embodiment, if theVF detection criteria are met (X_(VF)=VFNID), but the WZCS-CNT x meetsthe WZCS count threshold (indicating that x of the y QRS complexesexhibit high frequency content with respect to preceding baselinesegments), then final declaration of VF and delivery of the C/D shock ispostponed while a further number z of QRS complexes are examined. TheR-R intervals between ventricular sensed events and the morphologies ofthe further number z of QRS complexes are examined whereby the FDI countX_(VF) and the WZCS-CNT x are updated on a beat-to-beat basis. The finaldeclaration of VF and delivery of a C/D shock can only take place whenthe z R-R intervals occur, the z WZCS processing takes place, the VFdetection criteria continue to be met (X_(VF)=VFNID), and the WZCS-CNT xno longer meets the WZCS count threshold. The number z can be equal toor different than y. In a particular example, the VFNID is 18, Y=24, thematch count threshold is 6, y=8, and z=8. The methods of FIGS. 3 and 4employ WZCS signal processing as illustrated in FIG. 5, to derive theWZCS_(M) following the steps of FIG. 6 to distinguish fibrillatory(polymorphic) and monomorphic QRS complexes when VF detection criteriaare satisfied.

[0085] Turning first to FIG. 3, in step S300, the EGM amplitude iscontinually sampled, digitized, and temporarily stored in a buffer on aFIFO basis employing the VEGM amplifier 128 and ADC/MUX 104 in a mannerdescribed in the above-referenced '519 patent, for example. A VSENSEevent is declared in step S302 by the R-wave sense amplifier 126 or canbe determined from the temporarily stored EGM amplitude data. The R-Rinterval is calculated in step S304 when each VSENSE is declared, andthe R-R interval is compared to the FDI in step S306. In step S310, a“1” is shifted into the first stage of the VF counter, the data bits ofthe remaining stages are shifted one position, and the data bit in theYth stage is discarded when the R-R interval is shorter than the FDI asdetermined in step S306. In step S308, a “0” is shifted into the firststage of the VF counter, the data bits of the remaining stages areshifted one position, and the data bit in the Yth stage is discardedwhen the R-R interval is longer than the FDI as determined in step S306.The VF count X_(VF) can only be incremented when a “1” is shifted intothe first stage of the VF counter and a “0” is shifted out of the Ythstage in step S310.

[0086] In a preferred example, there are 24 VF counter stages, and theVFNID is set to a lesser number, e.g., 18. The stages containing “1”bits are counted to derive the VF count X_(VF). The VF count X_(VF) iscompared to a morphology stability test threshold (MS_(THRS)) in stepS310, wherein 0<MS_(THRS)<VFNID. As noted above, when VFNID=18 and y=8,the MS_(THRS) can be set to 8 or to 10 (18−8), for example.

[0087] The determination and storage of HPF data sets commences in stepsS314 and S316 when the VF count X_(VF) meets the MS_(THRS) as determinedin step S312. In S314, EGM data collected in step S300 preceding andfollowing the VSENSE event detected in step S302, e.g., a 200 ms windowbracketing the VSENSE event, is subjected to high pass filtering toderive the HPF data set for the current QRS complex. Step S314 isrepeated on each VSENSE event detected in step S302 and each time thatthe VF count X_(VF) meets the MS_(THRS) as determined in step S310. Thecurrent HPF data set calculated in step S314 is stored on a FIFO basisin the first stage of the HPF data register in step S316.

[0088] As described further below in reference to FIGS. 5 and 6, thefrequency content analysis is performed in step S318 to derive aWZCS_(M) value. The WZCS_(M) value is compared to a WZCS threshold(WZCS_(THRS)) in step S320. The WZCS-CNT x is maintained in a registerhaving y stages each set to “1” or “0” that can be implemented inhardware, firmware or software. The WZCS-CNT x is the number of “1”s,for example, in the register stages. Each time that the WZCS_(M) valuemeets the WZCS_(THRS) in step S320, a “1” is advanced into the firststage of the register, the contents of each stage is advanced to thenext stage, and the “1” or “0” in the yth stage is discarded in stepS322. Similarly, each time that the WZCS_(M) value does not meets theWZCS_(THRS) in step S320, a “0” is advanced into the first stage of theregister, the contents of each stage is advanced to the next stage, andthe “1” or “0” in the yth stage is discarded in step S324. Thus, thecount x is “incremented” if a “1” is advanced into the initial stage ofthe register and a “0” is discarded from the yth stage and “decremented”if a “0” is advanced into the initial stage of the register and a “1” isdiscarded from the Yth stage. The WZCS-CNT x remains the same if thesame bit value “1” or “0” is advanced into the initial stage of theregister and is discarded from the yth stage.

[0089] The WZCS-CNT x is compared to a match count threshold(WZCS-CNT_(THRS)) in step S326. Thus, when a sustained run of R-Rintervals shorter than the FDI occur, the steps S310-S326 can berepeated at least y times to derive a meaningful WZCS-CNT x. In stepS328, an evidence counter is set to a number z of QRS complexes when theWZCS-CNT x meets the WZCS-CNT_(THRS) in step S326. The evidence counteris decremented from z in step S330 each time that the WZCS-CNT x doesnot meet the WZCS-CNT_(THRS) in step S326. If the VF detection criteriaare met (X_(VF)=VFNID) in step S332, but the VT EVIDENCE-CNT exceedszero (indicating that x of the y QRS complexes exhibit high frequencycontent with respect to preceding baseline segments) as determined instep S334, then final declaration of VF is not made in step S338.Instead, VT is declared, and an appropriate VT therapy is delivered instep S336.

[0090] Thus, if both conditions of steps S332 and S334 are satisfied,then the ventricular tachyarrhythmia is finally declared to be amonomorphic fast VT. A fast VT therapy, e.g., a burst pacing therapy,may be delivered in step S336. It should be noted that in this algorithmof FIG. 3, further conventional morphological processing is preferablyconducted to discriminate SVT and VT so that such fast VT therapies arenot delivered to the ventricles during an SVT episode.

[0091] The ventricular tachyarrhythmia is declared to be VF and a C/Dtherapy is delivered in step S338 when the VF count X_(VF) meets theVFNID as determined in step S332 and when the VT EVIDENCE-CNT exceedszero as determined in step S334.

[0092] In practice, the FDI, the VFNID, and one or both of theWZCS_(THRS) and the WZCS-CNT_(THRS) can be varied by programming tooptimize the specificity of discrimination of true VF episodes in agiven patient. Moreover, ventricular tachyarrhythmia terminationalgorithms are followed to determine whether a delivered therapy hasterminated the episode. A C/D shock is typically delivered if theepisode is not terminated by a delivered VT therapy.

[0093] The method of FIG. 3 could be employed in a prophylactic ICD nothaving the capability of delivering a VT therapy in step S336 asindicated by the dashed line back to step S300. However, it may bedesirable to apply more rigorous criteria before delivery of a C/D shocktherapy is allowed as shown in FIG. 4. In FIG. 4, steps S400-S426 arefunctionally equivalent to steps S300-S326 of FIG. 3 as described above.A withhold delay corresponding to a VF withhold count (VF WITHOLD-CNT)of z VSENSE events is effectively turned on in step S428 when theWZCS-CNT x does meet the WZCS-CNT_(THRS) as determined in step S426before the VF count X_(VF) meets the VFNID as determined in step S432.In step S426, the WZCS-CNT x is compared to the WZCS-CNT_(THRS,) and theVF WITHOLD-CNT is set to z in step S428 when the WZCS-CNT x meets theWZCS-CNT_(THRS). In that circumstance, the C/D shock therapy cannot bedelivered until the WITHOLD-CNT is decremented from z back to zero instep S430 and the VF count X_(VF) still or again meets the VFNID asdetermined in step S432. The withhold delay z can additionally beprogrammed to optimize the specificity of discrimination of true VFepisodes in a given patient.

[0094] Thus, if the VF WITHOLD-CNT is set to z in step S428, then oneach subsequent repetition of steps S402-S424, the VF WITHOLD-CNT isdecremented each time that the WZCS-CNT x does not meet theWZCS-CNT_(THRS) as determined in step S426 or the VF WITHOLD-CNT isreset back to z each time that the WZCS-CNT x does meet theWZCS-CNT_(THRS) as determined in step S426. During this process, the VFcount X_(VF) can be incremented or decremented in steps S408 or S416.The ventricular tachyarrhythmia is declared to be VF and a C/D therapyis delivered in step S436 only when the VF count X_(VF) meets the VFNIDas determined in step S432 and when the VF WITHOLD-CNT is decremented tozero as determined in step S434. In practice, it would be expected thatthese steps would be met quickly during a true VF and that declarationand delivery of the C/D shock in step S436 would not be unduly delayed.

[0095] Turning to FIG. 5, it schematically depicts the determination ofthe WZCS_(M) morphologic value during NSR. The raw EGM illustratingdiscrete ORS complexes marked by a VSENSE event are depicted in tracing(a) of FIG. 5. As described above, the VSENSE event is derived by asense amplifier that is typically coupled to a bipolar pace/senseelectrode pair such that the VSENSE event is declared when the amplitudeof a near field EGM exceeds a sense threshold. In contrast, the depictedraw EGM is sensed by an EGM amplifier coupled to a unipolar or far fieldsense electrode pair such that a far field EGM is sensed. Therefore, thedepicted far field QRS complex commences prior to the declaration of theVSENSE event.

[0096] The raw EGM is sampled at a sampling rate, e.g., 128 Hz, anddigitized in steps S300 and S400. When the condition of steps S310 andS410, are met, the raw EGM samples are high pass filtered in steps S312and S314, respectively, to derive high pass filtered (HPF) EGM samplesas shown in tracing (b) of FIG. 5. The high pass filter function can beachieved using a 10^(th) order FIR high pass filter with a pole at 24Hz. The approximate center frequency for the ventricular sense amplifieris 24 Hz with −3 dB points at approximately 14 Hz and 41 Hz. The 24 Hzpole is approximately the center of this band and rejects the slowlyvarying components of the EGM samples during VF episodes. The high passfilter characteristics can be approximated by 2^(nd) order slopes(x(i+2)−2*x(i+1)+x(i)) in the original EGM samples illustrated intracing (a) of FIG. 5.

[0097] As shown in tracing (b) of FIG. 5, an event window is defined bya number of HPF samples or data points over a window duration, e.g., 11HPF sample or data points (sampled at 128 Hz) over, e.g. 85 ms,commencing prior to and following declaration of a VSENSE event. Abaseline window also comprising of 11 HPF samples or data points over 85ms is centered around the midpoint between the present VSENSE and theprevious VSENSE. During NSR, the HPF baseline data points would exhibitlittle change from absolute baseline or otherwise exhibit relatively lowamplitude. The determined HPF event and baseline data sets aretemporarily stored in steps S314 and S414.

[0098] The magnitude of each HPF data point is known, and therefore, thechange in direction (positive or negative) of the HPF waveform can bemathematically derived from a successive point subtraction of data pointmagnitudes. Each ZCP of the high pass filtered EGM signal with respectto the zero-magnitude line can be derived when there is a sign changebetween two successive sample points. The ZCP typically falls between aleading and a trailing (in time) HPF data point having differingpolarities or one of them is zero. The trailing sample point is denotedas the ZCP for simplicity. The absolute value of the slope of the HPFEGM signal is determined referenced to each ZCP by subtracting theleading and trailing HPF data points. Each ZCP is then associated withor weighted by the corresponding determined slope as illustrated intracing (b) of FIG. 5 by the vertical lines between peaks of the HPF EGMsignal.

[0099] As shown in tracing (c) of FIG. 5, the weighted ZCPs of thebaseline window are summed to provide a baseline WZCS_(B), and theweighted ZCPs of the VSENSE event window are summed to provide a VSENSEevent WZCS_(E). The entire tracing (c) of FIG. 5 is generated forillustrative purposed only by running a rolling window across eachsample point. However the algorithm only requires calculation of twosample points, one for the VSENSE event window and one for the precedingbaseline window for each ORS complex

[0100] Steps S318 and S418 are further illustrated in FIG. 6. StepsS602-S610 take place each time that a new HPF baseline data set isstored in step S314 or S414 as determined in step S600. Similarly, stepsS614-S622 take place each time that a new HPF event data set is storedin step S314 or S414 as determined in step S612. The order of stepsS600-S610 and steps S612-S622 can be reversed from that depicted in FIG.6.

[0101] In step S602, the ZCPs with respect to the absolute baselinewithin the baseline window data set are determined. The absolute valueof the slope of the high pass filtered EGM signal is determined at eachZCP in step S604, and each ZCP is weighted by the correspondingdetermined slope in step S606. In step S608, the weighted ZCPs of thebaseline window are summed to provide a baseline WZCS_(B) that istemporarily stored in step S610.

[0102] Similarly, in step S614, the ZCPs with respect to the absolutebaseline within the event window data set are determined. The absolutevalue of the slope of the high pass filtered EGM signal is determined ateach ZCP in step S616, and each ZCP is weighted as described above bythe corresponding determined slope in step S618. In step S620, theweighted ZCPs of the baseline window are summed to provide a VSENSEevent WZCS_(E) that is temporarily stored in step S622.

[0103] The WZCS_(M) is derived from the VSENSE event WZCS_(E) and thebaseline WZCS_(B) in step S624 by:

WZCS _(M) =WZCS _(E)+(WZCS _(E) −WZCS _(B))

[0104] The first component, WZCS_(E), estimates the frequency content ofthe VSENSE event window and the second component, (WZCS_(E)−WZCS_(B)),computes the difference between the frequency content during the VSENSEevent window and the preceding baseline window. In case of SVT andmonomorphic fast VT, where frequency content in the signals are highduring VSENSE events as compared to baseline, both these components willbe high giving a large value for WZCS_(M). Whereas, in case of VF, wherefrequency content in the signals are lower during VSENSE events ascompared to baseline, both these components are low adding up to a muchsmaller value for WZCS_(M). It also needs to be noted that the above twometric components can be used independently or in other mathematicalcombinations to achieve the same goal.

[0105] Further, the present invention is adaptable and applicable tosubcutaneous sensing and detection of arrhythmias.

[0106] This algorithm was tested on high rate episodes (RR intervalsfrom 220 ms-340 ms) using recorded episodes from earlier clinicaltrials. The test database consisted of 53 true VF episodes, 63 SVT(rapidly conducted AF/AFL) and 104 monomorphic fast VT episodes. For aparticular set of parameters for different thresholds, 51 out of 53 VFepisodes were detected as VF, 2 out 63 rapid SVT episodes were detectedas VF and 14 out of 104 monomorphic VT episodes were detected as VF.

[0107] Although the preferred embodiments described above relate to thediscrimination of ventricular tachyarrhythmias, it will be understoodthat the principles of the present invention may be applied todiscrimination of atrial tachyarrhythmias. For example, in practicewithin atrial or dual chamber ICDs, the near or far field atrial EGM andatrial sense events can be determined, AF detection criterion can bedefined, and an AF episode or atrial tachyarrhythmia (AT) can beprovisionally declared following the steps S300-S312 of FIG. 3 orS400-S412 of FIG. 4 or employing any other known AF detection criteria.The algorithm of FIG. 6 can be employed to determine a WZCS_(M) of theatrial EGM for use in the algorithms of FIG. 3 or FIG. 4 fordiscrimination between AF and organized AT. ATP therapies will bedelivered for organized atrial tachyarrhythmias only, and CD/shocktherapy may or may not be applied for AF detection. So, FIG.4 applies tothe case when only ATP therapies need to be delivered for organized ATand ATP therapy needs to be withheld for AF or disorganized AT. The stepS420 comparison can be reversed to perform an ATP therapy withhold ifevidence for AF or disorganized AT is found when WZCS_(M)<WZCS_(THRS).Therefore, it will be understood that the present invention can beapplied in discrimination of both atrial and ventriculartachyarrhythmias in the manner described herein.

[0108] Moreover, it will be appreciated that the present invention maybe practiced in contexts that do not rely upon the provisionaldeclaration of a polymorphic tachyarrhythmia, e.g., AF or VF, followingsteps S304-S312 and step S332 of FIG. 3 or steps S404-S412 and step S432of FIG. 4. The algorithm of FIG. 6 can be employed with the remainingsteps of FIGS. 3 or 4 to make the determinations of steps S336 and S338or step S436, respectively, based upon the results of step S334 or stepS434, respectively. Such a simplified algorithm can be advantageouslyapplied in discrimination of both atrial and ventriculartachyarrhythmias in the manner described herein.

[0109] Furthermore, it will be appreciated that the methods of thepresent invention for estimating frequency content can be used todiscriminate between a QRS complex and a T-wave and thus avoid T-waveoversensing problems.

[0110] All patents and publications referenced herein are herebyincorporated by reference in their entireties.

[0111] It will be understood that certain of the above-describedstructures, functions and operations of the above-described preferredembodiments are not necessary to practice the present invention and areincluded in the description simply for completeness of an exemplaryembodiment or embodiments.

[0112] In addition, it will be understood that specifically describedstructures, functions and operations set forth in the above-referencedpatents can be practiced in conjunction with the present invention, butthey are not essential to its practice. It is therefore to beunderstood, that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described withoutactually departing from the spirit and scope of the present invention.

[0113] One skilled in the art will appreciate that the present inventioncan be practiced with embodiments other than those disclosed. Thedisclosed embodiments are presented for purposes of illustration and notlimitation, and the present invention is limited only by the claims thatfollow.

What is claimed is:
 1. In an implantable medical device thatprovisionally detects a polymorphic tachyarrhythmia of the heart of apatient as a function of measured time intervals between sensed eventsin a cardiac signal, a method of improving the specificity ofdiscriminating between a monomorphic tachyarrhythmia and a polymorphictachyarrhythmia comprising the steps of: a) provisionally declaring apolymorphic tachyarrhythmia when at least a first number of the measuredtime intervals satisfy the polymorphic tachyarrhythmia detectioncriteria; b) successively sampling, processing, and temporarily storingthe cardiac signal to provide sampled signal amplitudes; c) determininga baseline window between sensed events and a sensed event windowencompassing the sensed event; d) determining baseline zero crossingpoints from the sampled signal amplitudes in the baseline window andevent zero crossing points from the sampled signal amplitudes in thesensed event window; e) determining the absolute signal slope at eachzero crossing point; f) weighting each zero crossing point by thedetermined absolute signal slope; g) summing the weighted zero crossingpoints in the baseline window to provide a baseline weighted zerocrossing sum; h) summing the weighted zero crossing points in the sensedevent window to provide a sensed event weighted zero crossing sum; i)subtracting the baseline weighted zero crossing sum from the sensedevent weighted zero crossing sum to provide a weighted zero crossing summetric related to the morphology of the cardiac signal; j) comparing theweighted zero crossing sum metric to a weighted zero crossing sumthreshold; k) accumulating a match count x of weighted zero crossing summetrics that meet the weighted zero crossing threshold over yrepetitions of steps b) through j) as long as a polymorphictachyarrhythmia is provisionally declared in step a); l) withholding thefinal declaration of a polymorphic tachyarrhythmia if the count xindicates that the corresponding cardiac signals exhibit frequencycontent suggestive of a monomorphic tachyarrhythmia; and m) making thefinal declaration of a polymorphic tachyarrhythmia if the count xindicates that the corresponding cardiac signals exhibit frequencycontent suggestive of a polymorphic tachyarrhythmia and the polymorphictachyarrhythmia is provisionally declared in step a).
 2. The method ofclaim 1, further comprising repeating steps b) through m) as long asstep a) is met until step m) is met.
 3. The method of claim 2, whereinthe implantable medical device further comprises an implantablecardioverter/defibrillator having the capability of delivering a C/Dshock therapy and further comprising: n) delivering a C/D shock therapywhen the final declaration of a polymorphic tachyarrhythmia is made instep m).
 4. The method of claim 2, wherein step l) comprises:establishing a withhold count corresponding to z measured time intervalsbetween sensed events of successive cardiac signals; and repeating stepsa) through k) at least ztimes.
 5. The method of claim 2, wherein theimplantable medical device further comprises an implantablecardioverter/defibrillator having the capability of delivering a C/Dshock therapy and wherein: step l) comprises establishing a withholdcount corresponding to z measured time intervals between each detectedfeatures successive cardiac signals; and further comprising: n)repeating steps b) through m) at least ztimes; and o) delivering a C/Dshock therapy when the final declaration of a polymorphictachyarrhythmia is made in step m).
 6. The method of claim 2, whereinthe implantable medical device further comprises an implantablecardioverter/defibrillator having the capability of delivering a C/Dshock therapy and delivering an anti-tachycardia therapy and furthercomprising: n) delivering a C/D shock therapy when the final declarationof a polymorphic tachyarrhythmia is made in step m); and o) deliveringan anti-tachycardia therapy when the count x indicates that thecorresponding cardiac signals exhibit frequency content suggestive of amonomorphic tachyarrhythmia in step l) and a polymorphic tachyarrhythmiais provisionally declared in step a).
 7. The method of claim 1, whereinthe implantable medical device further comprises an implantablecardioverter/defibrillator having the capability of delivering a C/Dshock therapy and delivering an anti-tachycardia therapy and furthercomprising: n) delivering a C/D shock therapy when the final declarationof a polymorphic tachyarrhythmia is made in step m); and o) deliveringan anti-tachycardia therapy when the count x indicates that thecorresponding cardiac signals exhibit frequency content suggestive of amonomorphic tachyarrhythmia in step l) and a polymorphic tachyarrhythmiais provisionally declared in step a).
 8. The method of claim 7, wherein:step k) comprises comparing the count x to a WZCS count threshold; andstep m) comprises making the final declaration of a polymorphictachyarrhythmia if the count x does not meet the WZCS count thresholdand a polymorphic tachyarrhythmia is provisionally declared in step a).9. The method of claim 1, wherein the implantable medical device furthercomprises an implantable cardioverter/defibrillator having thecapability of delivering a C/D shock therapy and further comprising: n)delivering a C/D shock therapy when the final declaration of apolymorphic tachyarrhythmia is made in step m).
 10. The method of claim9, wherein: step k) comprises comparing the count x to a WZCS countthreshold; and step m) comprises making the final declaration of apolymorphic tachyarrhythmia if the count x does not meet the WZCS countthreshold and a polymorphic tachyarrhythmia is provisionally declared instep a).
 11. The method of claim 1, wherein step l) comprises:establishing a withhold count corresponding to z measured time intervalsbetween sensed events of successive cardiac signals; and repeating stepsa) through k) at least ztimes.
 12. The method of claim 1, wherein theimplantable medical device further comprises an implantablecardioverter/defibrillator having the capability of delivering a C/Dshock therapy and wherein: step l) comprises establishing a withholdcount corresponding to z measured time intervals between each detectedfeatures successive cardiac signals; and further comprising: n)repeating steps b) through k) at least ztimes; and o) delivering a C/Dshock therapy when the final declaration of a polymorphictachyarrhythmia is made in step m).
 13. The method of claim 1, whereinstep k) comprises comparing the count x to a f WZCS count threshold; andstep m) comprises making the final declaration of a polymorphictachyarrhythmia if the count x does not meet the WZCS count thresholdand a polymorphic tachyarrhythmia is provisionally declared in step a).14. In an implantable medical device that provisionally detects apolymorphic tachyarrhythmia of the heart of a patient as a function ofmeasured time intervals between sensed events in a cardiac signal, asystem of improving the specificity of discriminating between amonomorphic tachyarrhythmia and a polymorphic tachyarrhythmiacomprising: provisional declaring means for provisionally declaring apolymorphic tachyarrhythmia when at least a first number of the measuredtime intervals satisfy the polymorphic tachyarrhythmia detectioncriteria; signal processing means for successively sampling, processing,and temporarily storing the cardiac signal to derive a plurality y ofdata sets of signal amplitudes related to y sensed events; windowdefining means for determining a baseline window between sensed eventsand a sensed event window encompassing the sensed event of each of the ydata sets; zero crossing point determining means for determiningbaseline zero crossing points from the sampled signal amplitudes in thebaseline window and event zero crossing points from the sampled signalamplitudes in the sensed event window; slope determining means fordetermining the absolute signal slope at each zero crossing point in thebaseline and sensed event windows; weighting means for weighting eachzero crossing point by the determined absolute signal slope; summingmeans for summing the weighted zero crossing points in the baselinewindow to provide a baseline weighted zero crossing sum and the weightedzero crossing points in the sensed event window to provide a sensedevent weighted zero crossing sum; means for subtracting the baselineweighted zero crossing sum from the sensed event weighted zero crossingsum to provide a weighted zero crossing sum metric related to themorphology of the cardiac signal; means for comparing the weighted zerocrossing sum metric to a weighted zero crossing sum threshold; countingmeans for accumulating a count x of weighted zero crossing sum metricsthat meet the weighted zero crossing threshold among y data sets as longas a polymorphic tachyarrhythmia is provisionally declared; withholdingmeans for withholding the final declaration of a polymorphictachyarrhythmia if the count x indicates that the corresponding cardiacsignals exhibit frequency content suggestive of a monomorphictachyarrhythmia; and final declaring means for making the finaldeclaration of a polymorphic tachyarrhythmia if the count x indicatesthat the corresponding cardiac signals exhibit frequency contentsuggestive of a polymorphic tachyarrhythmia and the polymorphictachyarrhythmia is provisionally declared.
 15. The system of claim 14,further comprising means for comparing the count x to a WZCS countthreshold; and the final declaring means further comprises making thefinal declaration of a polymorphic tachyarrhythmia if the count x doesnot meet the WZCS count threshold and a polymorphic tachyarrhythmia isprovisionally declared.
 16. The system of claim 15, wherein theimplantable medical device further comprises an implantablecardioverter/defibrillator having the capability of delivering a C/Dshock therapy and further comprising: means for delivering a C/D shocktherapy when the final declaration of a polymorphic tachyarrhythmia ismade.
 17. The system of claim 15, wherein: the withholding means furthercomprises: means for establishing a withhold count corresponding to zmeasured time intervals between sensed events of successive cardiacsignals if the count x indicates that the corresponding cardiac signalsexhibit frequency content; and means for decrementing the withhold counteach time that a count x indicates that the corresponding cardiacsignals exhibit frequency content suggestive of a polymorphictachyarrhythmia; and the final declaring means further comprises meansresponsive to the withhold count for declaring a polymorphictachyarrhythmia only when the withhold count is decremented to awithhold count less than z.
 18. The system of claim 17, wherein theimplantable medical device further comprises an implantablecardioverter/defibrillator having the capability of delivering a C/Dshock therapy and further comprising: means for delivering a C/D shocktherapy when the final declaration of a polymorphic tachyarrhythmia ismade.
 19. The system of claim 15, wherein the implantable medical devicefurther comprises an implantable cardioverter/defibrillator having thecapability of delivering a C/D shock therapy and delivering ananti-tachycardia therapy and further comprising: means for delivering aC/D shock therapy when the final declaration of a polymorphictachyarrhythmia is made; and means for delivering an anti-tachycardiatherapy when the count x indicates that the corresponding cardiacsignals exhibit frequency content suggestive of a monomorphictachyarrhythmia in step m) and a polymorphic tachyarrhythmia isprovisionally declared in step a).
 20. The system of claim 14, whereinthe implantable medical device further comprises an implantablecardioverter/defibrillator having the capability of delivering a C/Dshock therapy and further comprising: means for delivering a C/D shocktherapy when the final declaration of a polymorphic tachyarrhythmia ismade.
 21. The system of claim 14, wherein: the withholding means furthercomprises: means for establishing a withhold count corresponding to zmeasured time intervals between sensed events of successive cardiacsignals if the count x indicates that the corresponding cardiac signalsexhibit frequency content; and means for decrementing the withhold counteach time that a count x indicates that the corresponding cardiacsignals exhibit frequency content suggestive of a polymorphictachyarrhythmia; and the final declaring means further comprises meansresponsive to the withhold count for declaring a polymorphictachyarrhythmia only when the withhold count is decremented to awithhold count less than z.
 22. The system of claim 21, wherein theimplantable medical device further comprises an implantablecardioverter/defibrillator having the capability of delivering a C/Dshock therapy and further comprising: means for delivering a C/D shocktherapy when the final declaration of a polymorphic tachyarrhythmia ismade.
 23. The system of claim 14, wherein the implantable medical devicefurther comprises an implantable cardioverter/defibrillator having thecapability of delivering a C/D shock therapy and delivering ananti-tachycardia therapy and further comprising: means for delivering aC/D shock therapy when the final declaration of a polymorphictachyarrhythmia is made; and means for delivering an anti-tachycardiatherapy when the count x indicates that the corresponding cardiacsignals exhibit frequency content suggestive of a monomorphictachyarrhythmia in step m) and a polymorphic tachyarrhythmia isprovisionally declared.
 24. A method of processing a cardiac signal toderive sensed events and discriminating between a monomorphictachyarrhythmia and a polymorphic tachyarrhythmia comprising the stepsof: a) successively sampling, processing, and temporarily storing thecardiac signal to provide sampled signal amplitudes; b) determining abaseline window between sensed events and a sensed event windowencompassing the sensed event; c) determining baseline zero crossingpoints from the sampled signal amplitudes in the baseline window andevent zero crossing points from the sampled signal amplitudes in thesensed event window; d) determining the absolute signal slope at eachzero crossing point; e) weighting each zero crossing point by thedetermined absolute signal slope; f) summing the weighted zero crossingpoints in the baseline window to provide a baseline weighted zerocrossing sum; g) summing the weighted zero crossing points in the sensedevent window to provide a sensed event weighted zero crossing sum; h)subtracting the baseline weighted zero crossing sum from the sensedevent weighted zero crossing sum to provide a weighted zero crossing summetric related to the morphology of the cardiac signal; i) comparing theweighted zero crossing sum metric to a weighted zero crossing sumthreshold; j) accumulating a count x of weighted zero crossing summetrics that meet the weighted zero crossing threshold over yrepetitions of steps a) through i); and k) declaring a polymorphictachyarrhythmia if the count x indicates that the corresponding cardiacsignals exhibit frequency content suggestive of a polymorphictachyarrhythmia.
 25. A system of processing a cardiac signal to derivesensed events and discriminating between a monomorphic tachyarrhythmiaand a polymorphic tachyarrhythmia comprising: signal processing meansfor successively sampling, processing, and temporarily storing thecardiac signal to derive a plurality y of data sets of signal amplitudesrelated to y sensed events; window defining means for determining abaseline window between sensed events and a sensed event windowencompassing the sensed event of each of the y data sets; zero crossingpoint determining means for determining baseline zero crossing pointsfrom the sampled signal amplitudes in the baseline window and event zerocrossing points from the sampled signal amplitudes in the sensed eventwindow; slope determining means for determining the absolute signalslope at each zero crossing point in the baseline and sensed eventwindows; weighting means for weighting each zero crossing point by thedetermined absolute signal slope; summing means for summing the weightedzero crossing points in the baseline window to provide a baselineweighted zero crossing sum and the weighted zero crossing points in thesensed event window to provide a sensed event weighted zero crossingsum; means for subtracting the baseline weighted zero crossing sum fromthe sensed event weighted zero crossing sum to provide a weighted zerocrossing sum metric related to the morphology of the cardiac signal;means for comparing the weighted zero crossing sum metric to a weightedzero crossing sum threshold; counting means for accumulating a count xof weighted zero crossing sum metrics that meet the weighted zerocrossing threshold among y data sets; and means for declaring apolymorphic tachyarrhythmia if the count x indicates that thecorresponding cardiac signals exhibit frequency content suggestive of apolymorphic tachyarrhythmia.